1. Field of the Invention
The present invention relates to a driving control technique for a backlight that is provided to a liquid crystal display device or the like. More specifically, the present invention relates to an LED driving circuit for adjusting operations of an LED backlight, an LED driving method, and a liquid crystal display device.
2. Description of the Related Art
Recently, as an LCD (Liquid Crystal Display) backlight, an LED (Light Emitting Diode) has been used broadly from the viewpoints of long life, low power consumption, and reduction in the load on the environments (mercury free).
With an LED driving circuit on which the LED is mounted, generally known is a method which supplies an output voltage between an anode and a cathode by a booster circuit at the time of light-up, and supplies a current adjusted by a constant current circuit to the LED. With such method, the booster circuit is associated with the constant current circuit. That is, control of the output voltage is executed by feeding back the result acquired by monitoring the current by the constant current circuit to the booster circuit.
Further, as an LED dimming method, a current pulse dimming method (PWM dimming method) by PWM (pulse width modulation) is normally used for avoiding changes in the hues of the LED caused by changes in the current and based on the reason that it is easy to keep the correlativity of the relation between dimming and the luminance.
In the technical field related to the PWM dimming method, there is known a control circuit which has a function for prohibiting a boosting operation when the pulse is “Low”, i.e., when the LED is “OFF”. With this control circuit, no current is flown into the constant current circuit during a period where the LED is in an “OFF” state. This is a structure for avoiding an issue of excessive consumption of the electric power because the booster circuit turns to an overvoltage state when the booster circuit continuously boosts up for flowing the current to the LED under a feedback control.
However, with this control circuit, when the duty of the PWM is low, the influence upon the characteristic of the driving circuit and the display quality becomes great since the operation period of the booster circuit is short.
Japanese Unexamined Patent Publication 2011-228063 (Patent Document 1) and Japanese Unexamined Patent Publication 2009-238633 (Patent Document 2) are known as the techniques for overcoming such issues regarding the control circuit.
Patent Document 1 discloses a technical content which stores the output voltage in a period (PWM-ON period) where the LED array is lighted up as a reference voltage at the time of PWM dimming, holds the reference voltage even in a period (PWM-OFF period) where the LED array is turned off, and executes a boosting operation based thereupon so as to suppress drop of the applied voltage between the anode and the cathode and to suppress flicker of the LED caused due to the change in the voltage.
In order to avoid such an issue that a long time is required for boosting up to the optimum voltage between the anode and the cathode since the operating time of the booster circuit is extremely short at the time of starting up the power source when the duty ratio of the PWM dimming is small, Patent Document 2 discloses a technical content which shortens the time from the point at which the power is supplied to the point at which the LED is lighted up through providing a bypass circuit in addition to the LED circuit, operating the booster circuit (switching power source) with a pre-duty ratio set in advance via the bypass circuit without connecting to the LED circuit immediately after starting the power source, and switching to the LED circuit after boosted up to a prescribed voltage.
Further, as a technique for avoiding flash at the time of starting light-up in a case where the power source is started under PWM dimming, there is the technique disclosed in Japanese Unexamined Patent Publication 2012-243688 (Patent Document 3).
Regarding the dimming signal generating device disclosed in Patent Document 3, disclosed is a technical content for avoiding dimming of the LED with the duty ratio of 100% immediately after supply of the power source through providing a circuit which is capable of generating a pseudo PWM signal based on an alternating-current power source until the circuit for generating a PWM signal (a dimming signal generating circuit) starts an operation.
However, the dimming circuit according to Patent Document 1 executes a prescribed boosting operation in an OFF period based on the reference voltage held in an ON period right before, so that the voltage of the ON period right before can be maintained in the OFF period but the voltage cannot be boosted up than that of the ON period right before. That is, the boost-up time cannot be shortened greatly when it is desired to perform an operation for gradually increasing the anode voltage, e.g., when starting up the power source.
The LED driving circuit according to Patent Document 2 can shorten the boost-up time of the anode voltage. However, it is necessary to bypass the current for each of the parallel number of the LEDs in the structure of the bypass circuit and the power bypassed for each of the series number varies. Thus, the circuit structure becomes complicated and large-scaled as the number of the LEDs is increased, so that it is difficult to use the circuit in common.
Further, the current capacitance and the voltage rating equivalent to those of the LED circuit are required for the bypass circuit herein. Therefore, the bypass circuit is required for each type of the LED circuits, so that the circuit scale is increased.
Further, the LED driving circuit is structured to supply the current even in a period where the LED is not lighted up, so that there is such an issue that the power is excessively consumed in the no light-up period. In addition, there is a shift period in which the LED side and the detour route side (bypass side) are both in an ON state, so that there is a possibility of causing more excessive consumption of the power in the shift period.
Further, employed herein is a structure with which the bypass circuit and the LED circuit are switched in a voltage/current state where the LED is lighted up. Thus, there is a concern that flicker or the like may be visually recognized due to current ripple and voltage noise of the circuit generated at the time of switching or immediately after switching. That is, there is a possibility of causing display quality deterioration.
Furthermore, the technical content disclosed in Patent Document 3 is to generate a pseudo PWM signal based on the AC power source for avoiding flashing and flicker when starting the light-up of the device LED. In this case, it is effective for avoiding flashing if the pseudo signal is fixed depending on the external power source and the dimming duty ratio of the signal generating device is fixed and is higher than that of the pseudo PWM signal. However, as the LED driving circuit, it is inconvenient for a case where PWM is inputted as an external signal to change the duty ratio.
The present invention is for improving the inconveniences of the related techniques of the LED driving circuit. More specifically, it is an exemplary object of the present invention to provide an LED driving circuit and an LED driving method for preventing deterioration of the display quality such as flicker through effectively shortening the time required for starting up the LED backlight and stabilizing the driving operations thereof, and to provide a liquid crystal display device using the same.